1. Field of the Invention
The present invention relates to cosmetic and dermatological preparations for tanning the skin, in particular to those which also offer protection against UV radiation.
2. Discussion of Background Information
The harmful effect of the ultraviolet part of solar radiation on the skin is generally known. While rays having a wavelength of less than 290 nm (the UVC region), are absorbed by the ozone layer in the earth's atmosphere, rays in the range between 290 nm and 320 nm, the UVB region, cause erythema, simple sunburn or even burns of varying severity.
The erythema activity maximum of sunlight is given as the relatively narrow region around 308 nm.
Numerous compounds are known for protecting against UVB radiation; these are mostly derivatives of 3-benzylidenecamphor, of 4-aminobenzoic acid, of cinnamic acid, of salicylic acid, of benzophenone and also of 2-phenylbenzimidazole.
It is also important to have available filter substances for the range between about 320 nm and about 400 nm, the UVA region, since its rays can also cause damage. Thus, it has been found that UVA radiation leads to damage of the elastic and collagenous fibers of connective tissue, causing premature aging of the skin, and that it is to be regarded as a cause of numerous phototoxic and photoallergic reactions. The harmful effect of UVB radiation can be intensified by UVA radiation.
In addition, UVA radiation can cause skin damage by damaging keratin or elastin in the skin. This leads to a reduction in elasticity and water-storage capacity of the skin, i.e. the skin becomes less supple and tends towards wrinkling. The notably high incidence of skin cancer in regions where solar irradiation is strong indicates that damage to the genetic information in cells is also apparently caused by sunlight.
However, UV radiation can also lead to photochemical reactions, the photochemical reaction products intervening in the skin's metabolism.
Such photochemical reaction products are predominantly free-radical compounds, e.g. hydroxyl radicals. Undefined free-radical photoproducts which are formed in the skin itself can also display uncontrolled secondary reactions as a result of their high reactivity. However, singlet oxygen, a non-free-radical excited state of the oxygen molecule, can also arise during UV irradiation, as can short-lived epoxides and many others. Singlet oxygen, for example, differs from the normal triplet oxygen (free-radical ground state) by virtue of its increased reactivity. However, excited, reactive (free-radical) triplet states of the oxygen molecule also exist.
UV radiation is also a type of ionizing radiation. There is therefore the risk that ionic species may also arise during UV exposure, which then, for their part, are capable of oxidative intervention in the biochemical processes.
The pigmentation of human skin is essentially brought about by the presence of melanin. Melanin and its degradation products (melanoids), carotene, degree of perfusion, and the condition and thickness of the Stratum corneum and other skin layers permit skin shades from virtually white (in cases of reduced filling or in cases of an absence of blood vessels) or yellowish via pale brown-reddish, bluish to brown of different shades and finally almost black. The individual regions of the skin display differing depths of shade as a result of varying amounts of melanin.
Natural melanin protects the skin from penetrating UV radiation. The number of melanin granules produced in the melanocytes determines whether a person has pale skin or dark skin. In cases of strong pigmentation (e.g. in colored races, but also in those with pale skin following UV irradiation) melanin is also to be found in the Stratum spinosum and even in the Stratum corneum. It attenuates the UV radiation by up to about 90% before it reaches the corium.
As characteristic cell organelles, melanocytes contain melanosomes in which the melanin is formed. Upon excitation by UV radiation, inter alia, melanin is formed to an increased degree. This is transported via the living layers of the epidermis (keratinocytes) ultimately to the horny layer (corneocytes) and causes the more or less marked brownish to brown-black skin color. Melanin is formed as the final stage of an oxidative process in which tyrosine converts, with the assistance of the enzyme tyrosinase, via several intermediates to the brown to brown-black eumelanins (DHICA and DHI melanin) and/or, with participation of sulfur-containing compounds, to the reddish pheomelanin. DHICA and DHI melanins arise via the common intermediate stages dopaquinone and dopachrome. The latter is converted, partially with participation of further enzymes, either into indole-5,6-quinonecarboxylic acid or into indole-5,6-quinone, from which the two specified eumelanins form. The formation of pheomelanin proceeds, inter alia, via the intermediate products dopaquinone and cysteinyldopa.
Besides various functions of the skin's own melanin (also “detoxification”/binding of toxic substances/pharmaceuticals etc.), the function of melanin as a natural UV filter to protect against harmful UV rays, and the antioxidant function of melanin as protection against reactive oxygen species (oxidative stress), which may arise, inter alia, as a result of solar irradiation, is very important for skin, inter alia with regard to homeostasis, prevention of skin aging, prevention of sunburn etc. This thus gives rise not only to a cosmetic benefit in the sense of enhanced tanning as a result of the increased synthesis of melanin in the skin following topical application of the active ingredient according to the invention, but also an additional protection as a result of the various protective powers of melanin.
Depending on their sensitivity to light, the skin types below are normally differentiated:
Skin type Inever tans, always burns.Skin type IIrarely tans, burns easily.Skin type IIItans averagely well.Skin type IVtans easily to give a lasting tan, almost never burns.Skin type Vdark, often almost black skin, never burns.
The natural shielding from harmful UV radiation is a tangible advantage of natural skin tanning. Moreover, for many decades a “healthy” skin color has been a sign of, in particular, sporting activity and is therefore considered to be desirable by a broad section of consumers. Representatives of skin types I and II who wish to enjoy such a skin shade in any case therefore have to rely on self-tanning preparations. However, representatives of skin type III who do not wish to excessively be exposed to the risks of sunbathing but nevertheless want to appear tanned are also thankful target groups for self-tanning preparations.
Artificial skin tanning can be brought about in a cosmetic or medicinal way, the following approaches essentially playing a role:
The regular taking of carotene preparations results in carotene being stored in the subcutaneous fatty tissue, and the skin gradually turns orange to yellow-brown.
Using make-up preparations which can be washed off it is possible to achieve a slight skin shading (e.g. extracts of fresh green walnut shells, henna).
Coloring can also take place via the route of a chemical change in the horny layer of the skin using self-tanning preparations. The most important active ingredient is dihydroxyacetone (DHA). The skin tanning achieved in this way cannot be washed off and is removed only with the normal flaking of the skin (after about 10-15 days). Dihydroxyacetone can be referred to as ketotriose and reacts as a reducing sugar with the amino acids of the skin and the free amino and imino groups of keratin via a number of intermediates in the sense of a Maillard reaction to give brown-colored substances, so-called melanoids, which are sometimes also called melanoidins.
A disadvantage of tanning with dihydroxyacetone is that the skin tanned therewith is not protected from sunburn, in contrast to “sun-tanned” skin.
A further disadvantage of dihydroxyacetone is that, particularly under the influence of ultraviolet radiation, formaldehyde is eliminated, albeit in small amounts in most cases.
There was a need to find ways in which the decomposition of dihydroxyacetone can be effectively countered.
Coloration by means of self-tanning compositions takes place without exposure to sunlight. In contrast to this, so-called pre-tan products or tan promoters are also offered, which have to be applied prior to exposure to the sun. In the sun, a yellowing of these preparations then arises, which is said to lead to a slight brown-yellow coloration of the outer skin, which additionally enhances the “suntan”.
A further type of artificial tanning which is likewise completely independent of UV light can be brought about by the hormones which are usually released within the body also as a result of (natural) UV exposure and ultimately stimulate the melanocytes to synthesize melanin. In this connection, mention may be made, for example, of modifications of proopiomelanocortin (POMC), such as aMSH and synthetic variants (such as NDP), some of which have much higher activity than the natural aMSH. Although tanning can in principle be brought about by these hormones, their use in cosmetics is not possible since they are clearly pharmacologically effective substances (hormones) which should not be used widely without medicinal indication.
It was surprising and could not have been foreseen by the person skilled in the art that the use of one or more substances chosen from the group of pyrimidines and purines for boosting natural skin tanning and/or for stimulating melanogenesis in human skin may overcome the disadvantages of the prior art.